RFR: 8367981: Refine CompactHashtable comments

[Hamlin Li]

Hi,
Can you help to review this patch?

The comments of CompactHashtable and related classes are out of date, and some comments are unclear, wrong or misleading.
As the related classes are used in more and more scenarios, it's helpful to update the comments.

Thanks

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Commit messages:
 - intial commit
 - Merge branch 'openjdk:master' into master
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/27360/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27360&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367981
  Stats: 46 lines in 2 files changed: 9 ins; 4 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/27360.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27360/head:pull/27360

PR: https://git.openjdk.org/jdk/pull/27360